drivers: display: st7789v: fix gpio_dt_spec initialization

The `gpio_dt_spec` structures where never initialised, therefore the driver
would always fault with "E: Reset GPIO device not ready" printed to the
console.

Fixes: 069bf6be
("drivers: display: st7789v: use gpio_dt_spec")

Signed-off-by: Casper Meijn <casper@meijn.net>